In the P-1 study, tamoxifen was shown to prevent the development of invasive and in situ breast cancer. Raloxifene has shown to be an effective drug for the prevention of osteoporosis. It was observed in the MORE trial that there were fewer breast cancers in the group of patients that had taken raloxifene compared to the controls. This study will determine if raloxifene is either more or less effective than tamoxifen in reducing the incidence of invasive breast cancer in postmenopausal women who are at increased risk for the disease. A secondary goal is to determine whether raloxifene reduces the endometrial cancer rate compared to tamoxifen.